Implement a wrapper over Glib::Dispatcher that passes the arguments to
the signal consumer via synchronized `std::queue`.
Arguments are always passed by value and the return type of the signal
is expected to be `void`.

Add a fixture for writing tests that require interaction with Glib event
loop and a very basic test for SafeSignal.

Use `mode` (`waybar::Bar::setMode`) as a shorthand to configure bar
visibility, layer, exclusive zones and input event handling in the same
way as `swaybar` does.
See `sway-bar(5)` for a description of available modes.

This allows to apply the mode atomically and adds possibility of
defining custom modes (to be implemented).

Read `layer`, `exclusive`, `passthrough` into a special mode "default".
Drop `overlay` layer hacks, as it's easier to use `"mode": "overlay"`
for the same result.

…e" modes

Try to use the default bar id (`bar-0`) if none is set.

Intermittent CI failures without any useful diagnostics could be caused
by the OOM killer. 1024MB is not really enough to run 3 parallel jobs
with a modern C++ compiler.

Allow changing existing modes and adding new ones via `modes`
configuration key.
`modes` accepts a JSON object roughly described by the following type
```typescript
type BarMode = {
    layer: 'bottom' | 'top' | 'overlay';
    exclusive: bool;
    passthrough: bool;
    visible: bool;
};
type BarModeList = {
    [name: string]: BarMode;
};
```
and will be merged with the default modes defined in `bar.cpp`.

Note that with absence of other ways to set mode, only those defined in
the `sway-bar(5)`[1] documentation could be used right now.

[1]: https://github.com/swaywm/sway/blob/master/sway/sway-bar.5.scd